Ferguson, St Louis: Authorities in Missouri on Thursday stood by their earlier decision to withhold the name of the police officer who fatally shot an unarmed black teenager, but denied he was the person identified online by a hacker activist.

Demonstrators and the family of the victim, 18-year-old Michael Brown, have called on police to release the name of the officer, but officials have refused to do so, citing security concerns in a tense environment amid days of protests that have sometimes turned violent.

Among the concerns, they said, are online threats from the hacker group Anonymous, which has said it would release personal information about the police officer involved and on Thursday identified him by name in a Twitter post.

A device deployed by police goes off in the street as police and protesters clash on Wednesday night. Photo: AP

The St Louis County Police Department tweeted that the name given by Anonymous was of a man who is not an officer with the Ferguson police or the neighbouring St Louis County police.

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"Do not release more info on this random citizen," said a tweet from @stlcountypd.

"We can't let anonymous groups or even public groups pressure us into doing anything we don't think we should do," Edward Magee, spokesman for the St Louis County prosecutor's office, said.

A police officer aims his machine gun at demonstrators protesting the shooting death of teenager Michael Brown on Wednesday in Ferguson, Missouri. Photo: Scott Olson/Getty Images

A member of Anonymous, who goes by @The Anon Message on Twitter, said in an email that it was standing by the name it identified. "Of course, they'll deny," the person said, referring to the police in Missouri.

Protests racked this St Louis suburb for a fifth straight night Wednesday as anger flared anew over the police killing of an unarmed young black man.

This image taken from video, provided by journalist Wesley Lowery of The Washington Post, shows a police officer confronting Lowery in a fast-food restaurant in Ferguson. Photo: The Washington Post

Police sealed off the area that was the scene of vandalism and looting Sunday night.

In Ferguson, protesters have gathered every night since Brown was fatally shot on Saturday in the mostly black suburb of St Louis during what authorities said was a struggle over a gun in a police car. Some witnesses say he was outside the car with his hands up.

Police in Ferguson fired tear gas, stun grenades and smoke bombs on Wednesday night to disperse some 350 protesters, the fourth night of racially charged demonstrations after police shot to death an unarmed black teen.

A police officer patrols a business district on Wednesday in Ferguson, Missouri. Photo: Scott Olson/Getty Images

Some demonstrators hurled rocks at police as others scattered, while smoke engulfed the area. A Reuters reporter saw two young men preparing what looked like petrol bombs in a bus-stop shelter, their faces covered by bandanas. Police said protesters had thrown petrol bombs at officers.

Missouri Governor Jay Nixon was scheduled to visit north St Louis County on Thursday to assess the situation, and his office said he would also make an announcement. The governor has urged law enforcement agencies to respect the rights of the residents and the press.

Dozens of protesters have been arrested since Saturday. On Wednesday, two journalists, Wesley Lowery of the Washington Post and Ryan J. Reilly of Huffington Post, were arrested in a McDonald's restaurant and then released, while working in the area.

"And here in the United States of America, police should not be bullying or arresting journalists who are just trying to do their jobs to report to the American people about what they see on the ground."

"Now is the time for healing, peace and calm on the streets of Ferguson, and an open and transparent process to see that justice is done."

“We’ve done everything we can to demonstrate a remarkable amount of restraint,” St Louis County Police Chief Jon Belmar said in an interview outside the command post. “If there was an easy way to fix this, we would have already solved the problem.”

Officers had heard sporadic gunfire, he said. Two police officers were reportedly injured in the clashes. At least 10 people were arrested.

One of them was St Louis Alderman Antonio French, a friend of his, Liz Peinado, said in a Twitter post. During the unrest, French has posted videos on social media of protests and the police presence on the streets.

During the nighttime confrontation, protesters with shirts wrapped around their faces held signs that read, “Hands up, don’t shoot,” as police closed in on the crowd. The slogan has been adopted by protesters because witnesses said 18-year-old Michael Brown was running with his hands in the air when a policeman shot him to death Saturday in Ferguson.

In live amateur video posted to social media, police could be overheard telling the group to get out of the area or they would be arrested. Clouds of tear gas were visible in the background.

Tensions high

Protesters have said the lack of transparency by police investigating the incident - including the refusal to release the officer's name - have added to already high tensions. They have also called for St Louis County Prosecuting Attorney Bob McCullough to be removed from the case.

A protester shouts as she moves away from a line of riot police in Ferguson, St Louis. Photo: AP

Magee said the officer's name will be made public if he is charged. Even if he is not arrested, his name will be released after the investigation because of the high public interest, Magee added. The officer has been placed on administrative leave.

Anonymous, which has also called for nationwide protests over the situation in Ferguson, said it would release the officer's photo and other "damaging information" later Thursday.

Police have deployed camouflage-clad officers in body armor, including one manning a rifle on a tripod atop an armored car, to Ferguson.

A demonstrator, protesting the shooting death of teenager Michael Brown, walks away after being ordered off the street by police on Wednesday in Ferguson, St Louis. Photo: Scott Olson/Getty Images

"I've had enough of being pushed around because of the colour of my skin. I'm sick of this police brutality," said one protester, who gave only his first name, Terrell, 18. "I'm going to keep coming back here night after night until we get justice."

National figures from President Barack Obama to civil rights activist Reverend Al Sharpton have called for a peaceful response to the shooting.

Police Chief Thomas Jackson told reporters his priority was improving race relations in Ferguson, which has seen a stark demographic shift in recent decades, going from mostly white to mostly black. About two-thirds of the town's 21,000-strong population are black. On a police force of 53, three officers are black.